National Transportation Safety Board (NTSB) lead investigator Bill English, right, talks with investigator Roger Cox during an NTSB hearing in Washington, Tuesday, June 24, 2014, to establish the cause of Asiana Flight 214 airlines crash in San Francisco, and to make safety recommendations. (AP Photo/ Evan Vucci)

WASHINGTON - Mismanagement by the pilots of Asiana Flight 214, including confusion over whether one of the airliner's key controls was maintaining airspeed, caused the plane to crash while landing in San Francisco last year, the National Transportation Safety Board concluded Tuesday.

But the board also took the unusual step of faulting the complexity of the Boeing 777's autothrottle, as well as materials provided by the aircraft maker that fail to make clear under what conditions the automated system doesn't automatically maintain speed, saying they contributed to the accident.

The 777 has been in service 18 years and is one of the world's most popular wide-bodied airliners, especially for international travel. Until last year's accident, it had not been involved in a single fatal crash.

The board's acting chairman, Chris Hart, warned that the accident underscores a problem that has long troubled aviation regulators around the globe â€” that increasingly complicated automated aircraft controls designed to improve safety are also creating new opportunities for error.

The Asiana flight crew "over-relied on automated systems that they did not fully understand," Hart said.

"In their efforts to compensate for the unreliability of human performance, the designers of automated control systems have unwittingly created opportunities for new error types that can be even more serious than those they were seeking to avoid," he said.

The South Korea-based airline's pilot training also was faulted.

Boeing immediately rejected the notion that the 777s automated systems contributed to the accident, pointing to the aircraft's safety record.

"The auto-flight system has been used successfully for over 200 million flight hours across several airplane models, and for more than 55 million safe landings," the company said in a statement. "The evidence collected during this investigation demonstrates that all of the airplane's systems performed as designed."

The board didn't say that the autothrottle failed to perform as designed. But rather that its design, under certain circumstances, could lead to confusion as to whether it was controlling speed or in an inactive state.

The safety board "did the right thing," said Ilyas Akbari, a Los Angeles attorney representing 16 of Flight 214's passengers. "It took courage to call out Boeing because it's an American manufacturer" and one of the nation's largest employers and exporters.

But most of the fault lies with Asiana and its pilots, Akbari said.

Investigators said the flight's three veteran pilots made 20 to 30 different errors, some minor and others significant, during the landing approach on July 6, 2013.

Among the errors were that pilots didn't follow company procedures on calling out notifications about the plane's altitude, speed and actions they were taking during the landing approach. They also weren't closely monitoring the plane's airspeed â€” a fundamental of flying. Instead, they assumed the autothrottle was maintaining the required speed for a safe landing.

But the captain flying the plane, Lee Kang Kuk, 45, who was new to the 777, inadvertently deactivated the autothrottle, putting it into a hold mode. A training captain who was sitting next to Kuk in the right seat didn't notice the error, and then compounded it by turning off only one of two other key systems for managing the flight. Both systems are supposed to be on or off, but not one on and one off.

A third pilot riding in the jump seat noticed the plane was descending too fast but didn't speak up right away.

The plane, with 307 people on board, was too low and too slow as it neared the runway. Its tail struck a seawall and was ripped off. The rest of plane went spinning and sliding down the runway.

Despite the violence of the crash, only three people were killed. Two were Chinese teens seated in the back, who weren't wearing their seatbelts and were thrown from the plane. Investigators said if they had been wearing the belts they would most likely have survived.

A third teen was hit on the head by one of the plane's doors but survived the crash. She was killed when, lying unconscious, she was run over by two rescue vehicles in the chaos afterward. Nearly 200 people were injured.

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Mendoza reported from Santa Cruz, California.

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Follow Joan Lowy on Twitter at http://www.twitter.com/AP_Joan_Lowy and Martha Mendoza at http://www.twitter.com/mendozamartha

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